Attenuation probes are generally associated with oscilloscopes; however, they may be used as a means for coupling an input signal to any electronic test and measurement instrument.
An attenuator probe network typically comprises two resistors connected in series between the probe tip and ground, the resistance values of the resistors being in a predetermined ratio, and two capacitors connected in parallel with the two resistors respectively. The output of the probe is taken from the junction point of the resistors. Therefore, one of the resistors is connected in series between the probe tip and the probe's output terminal and the other resistor is a shunt resistor. Similarly, the capacitors comprise a series capacitor and a shunt capacitor. The ratio of the resistance values of the resistors establishes the DC attenuation factor of the network whereas the AC performance of the network is governed by the capacitance values of the capacitors. The impedance of a capacitor is frequency dependent, and therefore it is necessary to adjust the relative capacitance values of the probe network to provide a constant attenuation factor over the frequency range of the signals of interest. The operation of adjusting the relative capacitance values of the capacitors of an attenuation probe is known as compensation. An apparatus for indicating proper compensation of an adjustable attenuation probe network is described in U.S. Pat. No. 4,253,057. In the case of U.S. Pat. No. 4,253,057, the series resistor and the series capacitor are incorporated in the probe itself, while the shunt resistor is part of the instrument's input amplifier stage. The shunt capacitor comprises a fixed capacitor in the input amplifier stage and a variable capacitor in the probe itself. The apparatus described in U.S. Pat. No. 4,253,057 operates by applying a square wave signal to the probe network and comparing the peak value of the signal at the output terminal of the Probe network with a previously-detected and stored reference value.
A disadvantage of a Practical implementation of the apparatus described in U.S. Pat. No. 4,253,057 is that it is necessary for the operator to adjust the variable capacitor to its minimum capacitance value prior to making the final adjustment, because the adjustment must be made from the undercompensated state. Moreover, owing to charge redistribution when the compensation capacitor is adjusted, an erroneous voltage may be developed across the compensation capacitor, giving rise to false indications of compensation. Further, the stored reference value is detected by adjusting the compensation capacitor to the undercompensated state, and the reference value is subject to error due to the possibility of the voltage developed across the compensation capacitor being erroneous.